Monday, April 2, 2007

In recent weeks, skeptics of the notion that global warming is caused by humans have been promoting alternative theories. One of the most plausible explanations asserts that global warming is part of a natural cycle caused by changes in the temperature of the Sun. It's undeniable that the Earth experiences natural variations in climate due to the Sun and other factors, but the warming over the past 100 years has a fundamentally different character, and is unprecedented in the 600,000 year climate history that scientists have reconstructed from ice cores and tree rings.

One way to quantify the relative importance of natural variations compared to those caused by people is to calculate the climate "forcing" of each factor individually. In this way, scientists have shown that changes in the concentration of carbon dioxide (CO2) in the Earth's atmosphere are at least 5 times more important to global warming than any changes in the Sun. The skeptics argue that the atmospheric CO2 concentration is changing because of temperature changes on the Earth, rather than the other way around. In fact, scientists have shown that changes in the Earth's orbit over thousands of years lead to natural changes in global temperature (periodic ice ages) that also cause natural changes in the CO2 concentration. But this predictable effect cannot explain the extraordinary warming that the Earth has experienced in recent times.

Another point raised by skeptics is that for several decades after 1945, global temperatures were falling even though the atmospheric CO2 concentration was rising. This is true, but there is a very simple explanation: in addition to the warming effect of CO2, common pollution in the form of particulate matter called "aerosols" has a cooling effect on the climate. Prior to the clean-air legislation enacted in the 1960's and 1970's, the cooling from aerosols overwhelmed the warming from additional CO2. As developing countries such as China and India adopt similar clean-air measures, this may actually accelerate global warming in the future.

By raising plausible doubts about the responsibility of humans for global warming in modern times, skeptics are trying to confuse the public into inaction. If climate scientists successfully communicate these subtle effects to the public, the skeptics will ultimately fail.